Hello Leslie,
The MCU that is pin a pin compatible is the LPC4078 but the memory is also up to 512kB. LPC4078FBD100|ARM Cortex-M4|32-bit MCU|NXP
If you require more memory for the LPC1778 you can use a external memory, you can control it with the external memory controller peripheral (EMC).
For the LPC1773 you also have the SPI Flash interface (SPIFI). The SPI Flash Interface allows low-cost serial flash memories to be connected to the Cortex-M3 processor with little performance penalty compared to parallel flash devices with higher pin count.
Hope it helps!
Victor.
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